Social stimuli can greatly modify reproductive performance in mammals. In this proposal the influence of social stimulation occurring prior to birth and during the preweaning period on later reproductive performance will be examined in the house mouse. Two series of experiments will be conducted. In the first prenatal influences will be examined. The intra- uterine position of the fetus relative to its littermates can affect its later anatomy, physiology and behavior. Female fetuses located between two males in utero are masculinized compared to female fetuses without male neighbors in-utero. A good deal of laboratory work has been done on this phenomenon. We propose to examine whether such intro-uterine position can affect the later success of the individual in a wild population of mice contained on a segment of a highway interchange. A second series of experiments relates to the finding that mother mice consume significant quantities of their pup;'s urine. About 50% of the water in the mother's milk is water recovered from the pups in the litter. Mice show postpartum estrus and if they become pregnant the next litter goes into a diapause condition. The length of diapause is regulated by the duration and intensity of sucking by the pups. There is a close correlation between the sucking stimulus of the pups and the consumption of pup urine by mothers. We propose to test whether pup urine contains a pheromone which prolongs diapause. In addition, we will test whether diapause is shortened by the presence of adult males or their urine or lengthened by the presence of grouped females of their urine. Changes in diapause length will influence the inter-birth interval in rodents and have a significant effect on the rate of population growth.